Synthesis of extracellular gold nanoparticles using Cupriavidus metallidurans CH34 cells

In this study, spontaneous synthesis of a gold (Au) colloid using cells of Cupriavidus metallidurans CH34 is reported, and compared with results obtained using cells of the model bacterium Escherichia coli MG1655. To investigate the synthesis mechanism, bacterial biomass and secretomes from both strains were incubated with Au(III) ions. Only CH34 cells were capable of producing extracellular dispersions of Au nanoparticles (NPs). Transmission electron microscopy images showed that AuNPs morphology was dominated by triangular and decahedral nanostructures. Energy dispersive X-ray spectroscopy and Fourier transform infrared spectra showed the presence of sulphur and vibrations associated to proteins. Average AuNPs diameter was obtained by dynamic light-scattering measurements (DLS), NP tracking analysis measurements and analysis of electron microscopy images. Moreover, DLS measurements showed that the biogenic colloid was stable after exposure to ultrasound, high ionic strength and extreme pH conditions. The biogenic AuNPs produced by strain CH34 did not show antibacterial activity, in contrast to biogenic silver NPs. Comparative bioinformatic analysis of genomes from strain CH34 and strain MG1655 showed potential CH34 proteins that may be electron donors during reduction of Au(III) ions. On the basis of these results, a mechanism for the extracellular Au reduction by strain CH34 is proposed.